Does the "se" in line 65 refer to the act of precipitating himself into the pool, or does it refer somehow to him being armed ("armatum")? It is so close to the latter and so far from the former that it confuses me.

Next is the word "factam." Ørberg glosses this in his vocabulary supplement as the adjective "wrought." Its form is accusative feminine. It seems like it should mate with "voraginem", and it looks like it has verbal force.

My provisional translation for these lines would be:

"Finally a place in the middle of the forum was drained, which is called "Lake Curtius" from the name of a certain armed knight who, it is said, plunged [himself] headlong with his horse into a whirlpool [that was wrought] there."

If "se" relates to plunging, then "he plunged himself" (and his poor horse). If not it must mean something like "self-armed." And could "factam" have something to do with causing the whirlpool to come about (by his plunging?).

You've taken postremus to be an adverb, but you'll note that it does not have the form of an adverb (postremo) so it can't be so. I understand postremus to go with the locus: the final place.

You are correct in that the factam goes with the voraginem. The predicative nature of the participle is in full force here: into the whirlpool that had been formed or as Lovecraft would put it, spawned into existence.

Salve Timothee Last drained is/was a place in the middle of the forum which is called "Lake Curtius" after the name of a certain knight who is said to have thrown himself in his armour with his horse into the [drain] hole/pit made there.

About the distance between "se" and "praecipitavisse", in Latin there's a tendency for weak pronouns (those without any emphasis) to come in second position in their clause. I think there was a discussion here on this so others can probably explain this more.

'Weak Pronoun Positions' in Devine and Stephens, Latin Word Order (2006), pp. 282-287, wrote:So either the pronouns float syntactically and dock into second position for phonological reasons, or they are directly placed by the syntax just where we find them, namely in initial position discounting the superior conjunction...[p.283]If the domain of the weak pronouns in the examples in (174-175) is the infinitival clause, then the weak pronoun is first and not second in its domain...[p.284]In styles like that of Livy, which allow V-bar syntax, a weak pronoun can remain in the base verb phrase as a postverbal clitic, thus not raising at all; this is well attested for the reflexive(182) Multi et aliarum civitatium, qui Emporias perfugerant, dediderunt se; (Livy 34.16.5)... [pp.286,287]

Alright then. I had taken a look at that book and my post was an attempt to paraphrase where they say "As has been known for over a century, with various qualifications weak pronouns tend to appear in second position in the clause, that is after the first phrase or after the first word." Although a second look finds where they specifically discuss relative clauses, where they say "Similarly in relative clauses the weak pronoun usually raises to be left sister of the constituent in the scope of the relative phrase" which is not so clear to me but they give a number of examples like "quae tibi ex alio genere frumentum suppeditare possit (Verr 2.3.172)".

But in any case, I just wanted to point to tjnor that this kind of separation between a pronoun and its verb is not strange.